The UY21 is an indirectly heated half wave rectifier and dates from the early days of all glass construction in the early 1940's. By the late 1940's it had been superseded by the UY41 on the smaller B8A base. Reference 1040 incorectly lists the UY41 as a direct equivalent of the UY21.
The UY21 was designed to operate as part of a series heater chain in universal sets. The 100 mA current allowed a few valves to run from 240 Volts, eg a domestic radio. The maximum reservoir capacitor is 60 µF and the minimum series resistance has to be 175 Ohms. The design is quite efficient compared to octal valves of the same period.
Seen from the side the darkened anode is seen to have an X shape. The support rod for the getter is also used to radiate heat. The metal skirt and locating spigot are external to the all glass envelope.
The wide glass tube envelope is 26 mm in diameter and, excluding the B8B base pins and spigot is 75 mm tall.
References: Datasheet, 3002 & 1040. Type UY21 was first introduced in 1941.
Absolute Maximum Operating Conditions¶
Updated February 03, 2013.